Depreciate & remove mimetype

[ewels]

I don't think that mimetype is used by any tooling, and it generally confuses people about what to enter for it and what it will be used for.

The initial idea was that it could help 3rd party tooling for having generators - eg. if the mimetype is text/csv then a spreadsheet editor could be displayed for creating data. If it was text/json then a code editor and so on. However, in practice it's easy and arguably more robust to do this by using filename extension, plus no-one has added editor support anywhere (that I'm aware of).

As such, in order to keep the schema lean and not confuse people, proposal is to drop the field.